The invention relates to a circuit arrangement for demodulating signals, particularly frequency-modulated signals, in which a limiter, to whose input the signals to be modulated can be applied, is followed by a demodulator from whose output the demodulated signals can be derived.
Circuit arrangements as defined in claim 1 have been used for a long time in different receivers, for example, in radio receivers and are illustrated, for example, in the Valvo Handbook xe2x80x9cIntegrierte Analog-schaltungen fxc3xcr Rundfunk und NF-Anwendungen, 1981xe2x80x9d, p. 60. In addition to the desired demodulated output signal, the demodulator also generates unwanted residues of the carrier frequency or of the intermediate frequency. When demodulated signals are to be evaluated, whose frequencies are near these unwanted frequencies, the signal processing will be disturbed. Such a disturbance occurs, for example, in an FM demodulator of a car radio with a low intermediate frequency.
It is an object of the invention to suppress disturbing carrier residues at the output of the demodulator. This object is achieved by the circuit arrangement according to the invention in that the outputs of the demodulator and the limiter are connected to a respective input of a mixer whose output is connected to the input of the limiter via a low-pass filter.
The invention is based on the recognition that manufacture-conditioned spreads of parameters (offsets) of the components used cause the above-mentioned disturbances and generate an asymmetrical signal processing. These offsets can be largely reduced by the measures according to the invention. The residue of the intermediate frequency in the output signal is thus much smaller than in conventional circuits.
In accordance with a further embodiment of the circuit arrangement according to the invention, and dependent on the conditions imposed, it may be advantageous for a phase-equal mixing to connect the output of the limiter to the one input of the mixer via a delay device.
A further advantageous embodiment is characterized in that the limit frequency of the low-pass filter is below the frequency band of the signals to be demodulated.
Furthermore, it is advantageous when the signal to be demodulated is an intermediate frequency signal which can be applied via a bandpass filter.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.